One type of heat exchanger consists of a number of tubes through which a service fluid (normally a coolant) circulates and on the outside of which a process fluid (the fluid being cooled) flows. When both the service fluid and the process fluid are water, the heat exchanger is referred to as a water-to-water heat exchanger. In another common type of heat exchanger, the process fluid flows through a number of tubes, and a gas (frequently air) is circulated around the pipes, which often have fins attached to them to improve their heat transfer capabilities. When the process fluid is water, this type of heat exchanger is referred to as an air-to-water heat exchanger. Normally the process fluid is cooled in a heat exchanger, but there is no reason in principle that a heat exchanger cannot be used to heat a process fluid.
When a liquid such as water is used as the service fluid or the process fluid, the surfaces of the tubes that are in contact with the liquid may become fouled and the heat transfer efficiency of the device will therefore be impaired. (Fouling contamination is not usually a problem where only air contacts the tubes.) Fouling can take several forms: (i) particulate matter in the liquid may settle on or otherwise become attached to the surface of the tubes; (ii) substances dissolved in the fluid (e.g., calcium carbonate dissolved in water) may come out of solution and precipitate onto the heat transfer surfaces; (iii) the fluid may react with the heat transfer surface, forming a layer (e.g., corrosion on carbon steel) which acts as a barrier to the flow of heat; (iv) macroorganisms (e.g., Asiatic clams) or microorganisms (e.g., bacteria) may become attached to the tubes and thereby impede the heat flow between the process fluid and the service fluid. Microorganic fouling is a particular problem where the ultimate heat sink is an open body of water (an ocean, river or pond), and it is often more difficult to predict than the other kinds of barriers described above. Moreover, a layer of microorganisms may send out a layer of hairs or other projections to feed on nutrients in the water. These projections can impede the flow of the service fluid, producing a layer of relatively still water which acts as a further barrier to heat flow.
A nuclear power plant contains a number of heat exchangers which are designed to remove heat that may be generated during an emergency. Unless the plant actually experiences an emergency, these heat exchangers remain unused, and whether their heat removal capabilities have become impaired as a result of fouling is unknown. Recognizing the risks of this situation, the U.S. Nuclear Regulatory Commission on Jul. 18, 1989 issued Generic Letter 89-13, which requires that operators of nuclear power plants adopt a program to verify the heat transfer capability of all safety-related heat exchangers cooled by service water.
Because of the large volume of service and process water which flows through the heat exchangers used in nuclear power plants, testing the efficiency of such a heat exchanger presents problems. Whichever fluid (i.e., service or process) is to be heated (or chilled) to conduct the test, a temperature differential of several degrees (for example, 2 or 3 degrees F.) at most can be obtained. This is far less than the temperature differential that would occur during an actual emergency, and thus the behavior of the heat exchanger during an emergency can be predicted only by extrapolating the results of the test to a much larger temperature differential. Therefore, extremely accurate (and hence expensive) instruments must be used to avoid any errors of measurement that would be unduly magnified in the extrapolation process.
The efficiency of a heat exchanger can also be gauged relatively inexpensively by measuring the pressure drop between the inlet and outlet of the service water. The pressure drop is related to flow restriction which in turn reflects the amount of fouling, and for a particular exchanger and type of fouling this information can be used to estimate the heat transferability of the exchanger. However, this test is not very useful unless the operator develops a correlation between the pressure drop and the heat transfer rate of the particular exchanger involved. This in turn requires an accurate means of directly determining the heat transfer rate of the exchanger.
The difficulty of measuring the heat transfer performance of their heat exchangers has led some operators to clean them periodically, whether or not their performance is known to be impaired. While this is one solution to the problem, unnecessary cleaning may shorten the service life of a heat exchanger. Ideally, a heat exchanger should be cleaned only as often as is necessary to assure that its heat transfer capabilities are satisfactory.